Average current control for a switched power converter

ABSTRACT

A circuit for controlling a switch in a power converter in which peak current is regulated to achieve a specified average current through a load. Control logic is operable to monitor a voltage across a sensing resistor such that when the voltage across the sensing resistor reaches or exceeds a threshold value, the control logic generates a signal that causes a switch to be turned OFF.

CLAIM OF PRIORITY

This application is a continuation and claims priority to U.S. patentapplication Ser. No. 13/764,061, filed on Feb. 11, 2013, the entirecontents of which are hereby incorporated by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates to average current control for a switchedpower converter.

BACKGROUND

Switched power converters, such as Buck converters, can convert an inputpower to an output power so as to drive a load with specified voltage orcurrent values. There are various topologies to control the converteroutput. In some applications, such as light emitting diode (LED)lighting applications, it may be desirable to control the average outputcurrent flowing through the LED load.

Some converters include a control loop to regulate the average outputcurrent. The control loop may include sensing circuitry to sense theaverage output current, an error amplifier to amplify the signal fromsensing circuitry, and control logic to adjust the pulse widthmodulation (PWM) pulse duty cycle of the power switch. To ensure thatthe loop is stable, a compensation network typically is needed.Furthermore, because the loop has limited bandwidth (e.g., in the kHzrange), there sometimes is a delay in the output response when there isa sudden change of input power. These factors can limit the transientresponse of the power converter.

SUMMARY

The present disclosure describes circuits for controlling a powerconverter in which peak current is regulated to achieve a specifiedaverage current through a load. For example, in one aspect, a circuitincludes a first input to receive a voltage corresponding to apre-specified target average current through the load, and a secondinput to receive a voltage corresponding to a voltage across a sensingresistor immediately after the switch turns ON. The circuit generates athreshold value that is approximately equal to twice the voltagecorresponding to the pre-specified target average current through theload, minus the voltage corresponding to a non-zero voltage across thesensing resistor just after the switch turns ON. Control logic isoperable to monitor a voltage across the sensing resistor such that whenthe voltage across the sensing resistor reaches or exceeds the thresholdvalue, the control logic generates a signal that causes the switch to beturned OFF. The disclosure also describes a power converter thatincludes such a circuit.

In another aspect, the disclosure describes a method of controlling aswitch in a power converter in which peak current is regulated toachieve a specified average current through a load. The method includesreceiving a voltage corresponding to a target average current throughthe load, and receiving a voltage corresponding to a voltage across asensing resistor immediately after the switch turns ON. A signalrepresenting a threshold value is generated and is approximately equalto twice the voltage corresponding to the pre-specified target averagecurrent through the load, minus the voltage corresponding to the voltageacross the sensing resistor just after the switch turns ON. The methodincludes monitoring a voltage across the sensing resistor, andgenerating a signal that causes the switch to be turned OFF in responseto detecting that the voltage across the sensing resistor has exceededthe threshold value.

Some implementations can achieve various advantages, such as obviatingthe need for a conventional control loop and compensation circuitry.Thus, in some implementations, design and application complexity can bereduced. Furthermore, some implementations can have a fast transientresponse such that average current control can be achieved within oneswitching cycle.

Other aspect, features and advantages will be readily apparent from thefollowing detailed description, the accompanying drawings, and theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a switched power converter circuitcoupled to a load.

FIGS. 2A through 2D illustrate examples of waveforms in the circuit ofFIG. 1.

FIGS. 3A and 3B illustrate a first example of a circuit for generating athreshold value to control the average current through the load inaccordance with a user-specified value.

FIGS. 4A and 4B illustrate a second example of a circuit for generatinga threshold value to control the average current through the load inaccordance with a user-specified value.

FIG. 5 is a flow chart illustrating a method of regulating peak currentto achieve a specified average current through a load.

DETAILED DESCRIPTION

This disclosure describes average current control for a switched powerconverter in which peak current is regulated to achieve a desiredaverage current through a load. Although the techniques and circuitsdescribed here can be particularly useful where the load includes one ormore light emitting diodes (LEDs), they also can be used in connectionwith other applications in which current through a load needs to beregulated.

As illustrated in FIG. 1, a floating Buck converter 10 is electricallycoupled to one end of an inductive element (e.g., inductor L), which isconnected in series with a string of one or more LEDs 12 (i.e., theload). A diode 14 is coupled in parallel between a second end ofinductor L and the other end of the LED string. The other end ofinductor L also is coupled to the drain of a switching transistor M1.The source of transistor M1 is coupled to ground through a sensingresistor Rcs.

The node between the source of transistor M1 and sensing resistor Rcs iscoupled to control logic 16, which controls a switch driver 18 thatdrives the gate of transistor M1. When transistor M1 is ON, currentflows through sensing resistor Rcs. On the other hand, when transistorM1 is OFF, current flows through diode 14. As explained in greaterdetail below, control logic 16 uses a target average current value(AvgRef/Rcs) and a sensed low current value (ValRef/Rcs) flowing throughsensing resistor Rcs to obtain a peak current value (PkRef/Rcs), whereAvgRef, ValRef and PkRef represent voltage values. When the currentflowing through sensing resistor Rcs exceeds the peak current value(PkRef/Rcs), control logic 16 generates a control signal to causetransistor M1 to turn OFF. In particular, control logic 16 is operableto monitor the voltage across sensing resistor Rcs such that when thevoltage exceeds PkRef, the control logic generates a signal that causestransistor to turn OFF. As explained below, control logic 16 can controlthe turning OFF of transistor M1 so that the average current flowingthrough the LED load 12 is about equal to the target average currentvalue (AvgRef/Rcs), which can be chosen by the user.

The following paragraphs, together with FIGS. 2A-2D, describe examplesof waveforms for Buck converter 10. FIG. 2A illustrates an example ofthe waveform (e.g., voltage) on the gate of transistor M1. FIGS. 2B, 2Cand 2D illustrate, respectively, corresponding examples of voltagewaveforms in circuit 10, where I1 represents the current throughinductor L, I2 represents the current through sensing resistor Rcs, andI3 represents the current through diode 14. Thus, the waveform of FIG.2C represents an example of the voltage at the node between resistor Rcsand the source of transistor M1. This relative value of this voltage canbe measured, for example, using a comparator in control logic 16.

When switching transistor M1 is ON, current flows from node Vled throughLEDs 12, inductor L and sensing resistor Rcs to ground. In this case,the current I1 flowing through inductor L is equal to the current I2flowing through sensing resistor Rcs. When transistor M1 is OFF, currentflows from node Vled through LEDs 12, inductor L and diode 14 back tonode Vled. In this case, the current I1 flowing through inductor L isequal to current I3 flowing through diode 14. Control logic 16 controlswhether transistor M1 is turned ON or OFF.

When switch M1 turns ON, the current I1 flowing through sensing resistorRcs increases almost immediately from zero to ValRef/Rcs, which is thecurrent flowing through inductor L. The current I1 flowing throughsensing resistor Rcs increases substantially linearly until it reaches apeak value PkRef/Rcs, which triggers control logic 16 to turn OFF switchM1. To regulate the average current AvgRef/Rcs through sensing resistorRcs, which also is the average current flowing through inductor L andLED load 12, control logic 16 includes circuitry to generate a voltagevalue for PkRef that will cause the average current AvgRef/Rcs to takeon the desired value (e.g., a value set or specified by the user).

From FIGS. 2B through 2D, it is apparent that AvgRef=½*(ValRef+PkRef).Thus, PkRef=(2*AvgRef)−ValRef. As noted above, the average current valueAvgRef/Rcs (or the corresponding voltage value AvgRef) can beuser-defined and can be set to a value to which the current that flowsthrough the load is to be regulated. The ValRef value (i.e., the voltageacross sensing resistor Rcs immediately after switch M1 is turned ON)can be obtained, for example, by a sample-and-hold circuit in controllogic 16. The PkRef value can be obtained using circuitry that generatesa voltage value of [(2*AvgRef)−ValRef] as an output based on theuser-defined AvgRef value and the sensed ValRef value. The PkRef valuethen can be set as a threshold value of control logic 16 such that whenthe sensed voltage across resistor Rcs reaches (or exceeds) thethreshold value, it causes transistor M1 to be turned OFF. Control logic16 can include, for example, a comparator to compare the sensed voltageacross resistor Rcs to the threshold value PkRef. In this manner, theaverage current flowing through the LED load 12 will be substantiallyequal to the user-defined AvgRef/Rcs value. The duration for whichtransistor M1 remains OFF can be set, for example, to a fixed amount oftime using a RC circuit or digital circuitry.

FIG. 3A illustrates a first circuit 20 for generating a PkRef voltagevalue based on a user-defined AvgRef voltage value and a sensed ValRefvoltage value. Depending on the implementation, a user can specify thetarget average current either by directly specifying the desired averagecurrent value (AvgRef/Rcs) or by specifying the corresponding voltagevalue (AvgRef). In the illustrated example, the user-defined AvgRefvalue is provided to the positive (+) input of a first amplifier 22, andthe sensed ValRef value is provided to the positive (+) input of asecond amplifier 24.

The output of first amplifier 22 is coupled to the gate of an NMOStransistor M2, whose source is coupled to ground through a resistor R3.The node between the source of transistor M2 and resistor R3 is coupledto the negative (−) input of first amplifier 22. The drain of transistorM2 is coupled to a first current mirror formed by PMOS transistors M4and M5. The output of the first current mirror (i.e., the drain oftransistor M5) is coupled to ground through resistor R6.

The output of second amplifier 24 is coupled to the gate of an NMOStransistor M11, whose source is coupled to ground through a resistorR12. The node between the source of transistor M11 and resistor R12 iscoupled to the negative (−) input of second amplifier 24. The drain oftransistor M11 is coupled to a second current mirror formed by PMOStransistors M9 and M10. The output of the second current mirror (i.e.,the drain of transistor M9) is coupled to a third current mirror formedby NMOS transistors M7 and M8. The output of the third current mirror(i.e., the drain of transistor M7) is coupled to the output of thesecond current mirror (i.e., the drain of transistor M5). The PkRefvalue is obtained from the node between the first and third currentmirrors (i.e., the node connecting the drains of transistors M5 and M7).

In FIG. 3A, the values “1×’ and “2×” indicate the relative sizes of thetransistors and the relative sizes of the resistors. Thus, for example,transistors M4, M7, M8, M9 and M10 have approximately the same size,whereas transistor M5 is about twice as large. Likewise, resistors R3,R6 and R12 can have substantially the same resistance as one another,which in the illustrated example is assumed to be R. The circuit 20generates an output voltage PkRef equal (or about equal) to[(2*AvgRef)−ValRef].

FIG. 3B illustrates further details of operation of the circuit of FIG.3A. In particular, FIG. 3B illustrates various currents that aregenerated in the circuit. For example, first amplifier 22, transistor M2and resistor R3 collectively generate current I4, where I4=(AvgRef/R).First current mirror (i.e., transistors M4 and M5) generates current I5,where I5=2*I4=2*(AvgRef/R).

Additionally, second amplifier 24, transistor M11 and resistor R12collectively generate current I6, where I6=ValRef/R. Second currentmirror (i.e., transistors M9 and M10) generates current I7, where I7=I6,and third current mirror (i.e., transistors M7 and M8) generates currentI8, where I8=I7=I6=ValRef/R. Transistors M5 and M7 collectively generatecurrent I9, where I9=I5−I8=(2*AvgRef−ValRef)/R. The voltage acrossresistor R6 is thus PkRef=I9*R=2*AvgRef−ValRef.

The voltage value PkRef from FIG. 3A (or FIG. 3B) can be used by controllogic 16 as the threshold value to determine when to turn off transistorM1 (see FIG. 1). In particular, as explained above, when control logic16 detects that the voltage across sensing resistor Rcs reaches thevalue PkRef, control logic 16 turns off transistor M1, thus ensuringthat the average current flowing through the LED load 12 issubstantially equal to the user-defined target AvgRef/Rcs value.

FIG. 4A illustrates a second circuit 30 for generating a PkRef voltagevalue based on a user-defined AvgRef voltage value and a sensed ValRefvoltage value. Here too, depending on the implementation, a user canspecify the target average current either by directly specifying thedesired target average current value (AvgRef/Rcs) or by specifying thecorresponding voltage value (AvgRef). In the illustrated example, theuser-defined AvgRef value is provided to the positive (+) input of afirst amplifier 22, and the measured ValRef value is provided to thepositive (+) input of a second amplifier 24. The implementation of FIG.4A uses only two current mirrors instead of three, as in theimplementation of FIG. 3A.

As shown in FIG. 4A, the output of first amplifier 22 is coupled to thegate of an NMOS transistor M22, whose source is coupled to groundthrough a resistor R23. The node between the source of transistor M22and resistor R23 is coupled to the negative (−) input of first amplifier22. The drain of transistor M22 is coupled to a first current mirrorformed by PMOS transistors M24 and M25. The output of the first currentmirror (i.e., the drain of transistor M25) is coupled to ground throughresistor R28.

The output of second amplifier 24 is coupled to the gate of an NMOStransistor M27, whose source is coupled to ground through a resistorR26. The node between the source of transistor M27 and resistor R26 iscoupled to the negative (−) input of second amplifier 24. The drain oftransistor M27 is coupled to a second current mirror formed by PMOStransistors M29 and M30. The output of the second current mirror (i.e.,the drain of transistor M30) is coupled to a node between the drains oftransistors M22 and M24. The PkRef voltage value can be obtained fromthe node connecting the output of the first current mirror and resistorR28.

In FIG. 3A, the values “1×’ and “0.5×” indicate the relative sizes ofthe transistors and the relative sizes of the resistors. Thus, forexample, transistors M24, M25, M29 and M30 have approximately the samesize. On the other hand, resistor R23 has about half the resistance asresistors R26 and R28, which in the illustrated example is assumed to beR. The circuit 30 generates an output voltage PkRef equal to about[(2*AvgRef)−ValRef].

FIG. 4B illustrates further details of operation of the circuit of FIG.4A. In particular, FIG. 4B illustrates various currents that aregenerated in the circuit. For example, first amplifier 22, transistorM22 and resistor R23 collectively generate current I10, whereI10=(2*AvgRef/R). Second amplifier 24, transistor M27 and resistor R26collectively generate current I11, where I11=ValRef/R. Additionally,second current mirror (i.e., transistors M29 and M30) generates currentI12, where I12=ValRef/R.

As can be seen from circuit 30 in FIG. 4B, current I10=I12+I13. Thus,current I13=I10−I12=(2*AvgRef−ValRef)/R. Furthermore, the current I14 atthe output from the first current mirror is the same as I13. Thus,I14=(2*AvgRef−ValRef)/R, and the voltage across resistor R28 isPkRef=(2*AvgRef−ValRef).

The voltage value PkRef from FIG. 4A (or FIG. 4B) can be used by controllogic 16 as the threshold value to determine when to turn off transistorM1 (see FIG. 1). In particular, as explained above, when control logic16 detects that the voltage across sensing resistor Rcs reaches thevalue PkRef, control logic 16 turns off transistor M1, thus ensuringthat the average current flowing through the LED load 12 issubstantially equal to the user-defined target value (AvgRef/Rcs).

The foregoing circuits can thus be used in a method of controlling aswitch in a power converter in which peak current is regulated toachieve a specified average current through a load. As indicated by FIG.5, the method can include receiving a voltage corresponding to a targetaverage current through the load (block 100) and receiving a voltagecorresponding to a voltage across a sensing resistor immediately afterthe switch turns ON (block 102). The method further includes generatinga signal representing a threshold value that is approximately equal totwice the voltage corresponding to the target average current throughthe load, minus the voltage corresponding to the voltage across thesensing resistor just after the switch turns ON (block 104). A voltageacross the sensing resistor is monitored (block 106), and a signal isgenerated to cause the switch to be turned OFF in response to detectingthat the voltage across the sensing resistor has exceeded the thresholdvalue (block 108).

The described circuits thus can be used to control the average currentto make it highly accurate. Compared to controlling the peak current,controlling the average current can, in some implementations, result ina more accurate and uniform average current value, despite variations inthe values of external components. When controlling the peak current,the average current will tend to be lower than the controlled peakcurrent by half of the ripple current (i.e., PkRef−ValRef), which mayvary significantly because it depends on multiple parameters, such asswitching frequency, external inductor value and input voltage. Incontrast, the techniques described above can, in some cases, achieve anaverage current that varies little, if at all, with different switchingfrequencies, external inductor values or input values.

Although the foregoing examples are described in connection with afloating Buck converter, the techniques and circuits described here canbe used with other types of switched power converters as well (e.g.,forward converters). Also, although the circuits of FIGS. 3A and 4Ainclude MOS transistors, other implementations can use other types oftransistors (e.g., bipolar transistors).

Some implementations can achieve various advantages, such as obviatingthe need for a conventional control loop and compensation circuitry.Thus, in some implementations, design and application complexity can bereduced. Furthermore, some implementations can have a fast transientresponse such that average current control can be achieved within oneswitching cycle.

Other implementations are within the scope of the claims.

What is claimed is:
 1. An apparatus comprising: a load; a switch havingfirst, second and third terminals, the first terminal coupled to theload, the second terminal coupled to a sensing resistor, and the switchconfigured such that a signal applied to the third terminal opens theswitch; a switch driver having an output coupled to the third terminal;and control logic operable, in response to a voltage across the sensingresistor exceeding a threshold value, to cause the switch driver togenerate the signal, the threshold value based, at least in part, on avalue corresponding to a target average current through the load and ona voltage across the sensing resistor, the signal opening the switchsuch that an average current flowing through the load is about equal tothe target average current value, wherein the control logic is operableto generate the threshold value and comprises: a first input to receivea voltage corresponding to the value that corresponds to the targetaverage current; a second input to receive a voltage corresponding to avoltage across the sensing resistor just after the switch is closed; andmeans to generate, in response to signals at the first and secondinputs, a voltage corresponding to the threshold value.
 2. The apparatusof claim 1 wherein the load comprises at least one LED.
 3. The apparatusof claim 1 wherein the target average current through the load is auser-defined value.
 4. The apparatus of claim 1 wherein the means togenerate the voltage corresponding to the threshold value includes aplurality of current mirrors.
 5. The apparatus of claim 1 wherein thecontrol logic is operable to monitor the voltage across the sensingresistor.
 6. A method of controlling a switch to achieve a specifiedaverage current through a load, the method comprising: receiving a firstvoltage corresponding to a target average current through the load;receiving a second voltage corresponding to a voltage across a sensingresistor just after the switch is closed; generating a threshold voltagebased, at least in part, on the first voltage and the second voltage;monitoring a voltage across the sensing resistor; and generating asignal to cause the switch to be opened in response to detecting thatthe voltage across the sensing resistor has exceeded the thresholdvoltage.
 7. The method of claim 6 including causing the switch toalternate between its open and closed states such that an averagecurrent flowing through the load is approximately equal to the targetaverage current value.
 8. The method of claim 6 wherein the thresholdvalue is approximately equal to a difference between twice the firstvoltage and the second voltage.
 9. The method of claim 6 wherein thetarget average current through the load is user-defined.
 10. The methodof claim 6 wherein the load includes at least one LED.
 11. The method ofclaim 6 including causing the switch to be closed after a fixed amountof time.